// 成绩转换

#include <stdio.h>

int main(int argc, char const *argv[])
{
    int score;

    // printf("Input score:\n")
    scanf("%d", &score);

    switch (score / 10)
    {
        case 10:
        case 9:
            printf("A\n");
            break;
        case 8:
            printf("B\n");
            break;
        case 7:
            printf("C\n");
            break;
        case 6:
            printf("D\n");
            break;
        default:
            printf("F\n");
            break;
    }

    return 0;
}